As is known in the art, a photovoltaic generator (PVG) comprises a plurality of photovoltaic cells (PVs) connected in series and/or in parallel. A photovoltaic cell is a semiconductor diode (p-n or p-i-n junction) designed to absorb light energy and convert it into electrical power. When photons are absorbed by the semiconductor, they transfer their energy to the atoms of the p-n junction so that the electrons of these atoms are freed and create free electrons (n-type charge) and holes (p-type charge). A potential difference then appears between the two (p-type and n-type) layers of the junction. This potential difference can be measured between the positive and negative terminals of the cell. The maximum voltage of a cell is typically about 0.6 V for zero current (open circuit) and the maximum current that can be delivered by the cell is highly dependent on the level of sunlight received by the cell.
The expression “tandem junction photovoltaic cell” denotes a muitijunction cell consisting of two simple junctions stacked one on top of the other so as to increase the bandwidth of the solar spectrum absorbed by the cell. Depending on the technology, the two junctions may be in direct contact with each other or in indirect contact via an intermediate film of transparent conductive oxide. In the latter case, the transparent conductive oxide intermediate between the two junctions acts as an intermediate reflector for increasing the optical path length of the light via multiple reflections.
FIG. 1, for example, shows a schematic of a tandem cell composed of a first junction made of amorphous silicon (a-Si:H) and a second junction made of microcrystalline silicon (μc-Si:H) in direct contact in cross section along the path of the incident light. The relative thicknesses of the various films have not been shown to scale in FIG. 1. The various materials are deposited as thin films on a glass substrate 10 by PVD (physical vapor deposition) or PECVD (plasma enhanced chemical vapor deposition). The following are thus deposited in succession: a first transparent conductive electrode 11, a first simple p-i-n junction 15 forming the front photovoltaic cell, a second simple p-i-n junction 16 forming the back photovoltaic cell, a second transparent conductive electrode 12 and a back reflector 20. For practical reasons relating to manufacture, tandem-cell architectures are at the present time mainly produced in what is called thin-film technology, whether the cads are inorganic, organic or hybrid (inorganic/organic). In thin-film technologies, the physical superposition of the photovoltaic cells is achieved by depositing in succession appropriate sequences of electrodes 11, 12, for collecting the current produced, and active films 15, 16.
Tandem cells are considered to be a key advanced technology in the photovoltaic device fields mainly because of their electrical conversion efficiencies. Specifically, production of tandem architectures consists in physically superposing (relative to the expected direction of incident light) two photovoltaic cells having respective optical absorption bands that are shifted in energy. Optically coupling the cells provides the array (i.e. the tandem) with an overall absorption bandwidth that is higher than that of the separate cells. The electrical conversion efficiency is thereby increased providing that this optical absorption can be converted into electrical energy and extracted.
FIG. 2, for example, is a plot illustrating the conversion efficiency expressed in percentage (%) for a tandem cell made of thin silicon films. The respective absorption bands of the superposed cells (“upper cell” for the front cell and “lower cell” for the back cell) and the overall absorption band of the cell (“superposition”) are shown. Tandem-cell technology is one way of increasing the energy performance of photovoltaic generators. Various tandem-cell architectures have thus been developed in the last few years. The reader may refer for example to documents EP-A-1 906 457, US-A-2008/0023059 or WO 2004/112161. These documents each provide various assemblies of photovoltaic materials aiming to increase the energy absorbed by the array.
The tandem cells described above are characterized by a double coupling: optical coupling due to the stack of active photovoltaic cells in various bands of the solar spectrum; and electrical coupling via direct or indirect contact of the two junctions and the presence of two electrodes at the ends of the tandem.
A major drawback of the electrical coupling of a tandem cell is that the currents generated by the photovoltaic cells forming the tandem need to match, whatever the solar conditions. This ideal case is in fact not possible because the current generated by each cell intentionally depends on the region of the spectrum in which they are active and varies depending on the solar conditions. This means that the tandem cell is intrinsically limited by the weakest of its elements. Such a limitation on current greatly reduces the theoretical efficiency of a tandem cell.
It has therefore been proposed to electrically decouple the junctions of a tandem cell. The photovoltaic cells of the tandem are still optically coupled but are electrically decoupled. Each junction is associated with two electrical electrodes and thus a tandem photovoltaic cell is obtained having four electrodes, two electrodes for each of the two tandem junctions. A film of material that is transparent to light and electrically insulating is inserted between the electrodes of adjacent junctions.
The electrodes of the tandem cell are, in general, electrically connected by way of current output terminals, via a junction box, to en electronic device for converting a direct-current (DC) voltage into an alternating current (AC) voltage compatible with the mains grid. This device also allows the array of photovoltaic cells to be controlled, or even each of the cells to be controlled independently. The two current output terminals of a photovoltaic cell are, in general, located either on opposite sides of the photovoltaic cell in two junction boxes, or in the center of the cell in a single junction box. FIG. 1 of U.S. Pat. No. 4,461,922, for example, shows two superposed tandem cells forming a module having current output terminals located on opposite sides of the module. Control of the module therefore requires that two junction boxes be placed on opposite faces of the module. Arranging junction boxes on opposite sides of the module has the drawback of making the assembly consisting of the module and the junction boxes bulky.
Furthermore, when two identical photovoltaic cells are directly superposed, the current output terminals are separated only by a very small distance, for example equal to the thickness of the film of insulating material that is transparent to light and intermediate between two adjacent photovoltaic cells. This thickness is about a millimeter or less. Superposition of these photovoltaic cells therefore implies superposition of electrical contact strips belonging to each of the two mils and the risk of short-circuits within the 4-wire photovoltaic cell formed. In addition, access to the electrodes is made difficult because of the small space separating the electrodes of a given polarity located in two adjacent photovoltaic cells. It is therefore difficult to connect them to a junction box.
There is therefore a need for a multijunction and multiterminal photovoltaic device in which the risk of short-circuits between the current-collecting strips of each of the cells is as small as possible and which can be controlled via a single junction box. In particular, there is a need for a method for manufacturing a multijunction photovoltaic device that makes connecting the current output terminals of each photovoltaic cell to the junction box easier.